1. Field of the Invention
This invention relates in general to noise suppression, and more particularly to a method and apparatus for removing noise spikes from an input transducer (head) in a storage system.
2. Description of Related Art
Magnetic data is increasingly stored at higher and higher densities which in turn require increases in the sensitivity of the transducers (heads) used to read the recorded signals. Noise reduction and cancellation likewise becomes more important as sensitivity of the heads increases. Recently, a previously unreported phenomenon associated with data readback has been identified that detrimentally affects the error rate in a data storage system. Errors in data readback have been identified to be caused by very sharp spikes at the input (channel) preamplifier. The sharp spikes are of electrical origin and have been observed in systems using MR heads. The spikes which can be considered to be impulse noise are very narrow in width and broad in spectral content, can be of either polarity and are distributed over a wide range in amplitude.
The effect of the sharp spikes in the input channel signal is randomly dispersed single bit errors. This effect is intermittent and can degrade the soft error rate to unacceptable levels. In fact, spikes above a certain amplitude have sufficient energy in the bandwidth of the recording channel to interfere with detection of the media signal. The precise origin of these spikes is not well understood. It appears that they can occur in the absence a recording signal or MR bias current, but appear to require media/head contact and their relative motion.
FIG. 8 illustrates a graph 800 of a set of noise spikes 802 detected on a DC erased track with an oscilloscope set to persistence mode and a positive trigger level. Note that the noise spikes 802 are identical in structure, i.e., they have substantially the same pulse width 810, but vary greatly in amplitude 812.
As a result of such phenomenon, a number of tracks on head arrays may suffer seriously degraded error rates. Problem tracks are randomly distributed on heads, and the problem is intermittent with random increase and decreases in the occurrence of the phenomenon. In addition, problem tracks have shown noise spikes with some media and not with others.
FIG. 9 illustrates a graph 900 of an noise spike 910 corrupting a channel signal 920 in a low end system. As in the high end, the noise spikes 910 are identical in structure, but vary greatly in amplitude. Here too the noise spikes 910 may be found to have a positive and negative (not shown) polarity and to be intermittent.
Since noise spikes are not written to the tape during the recording process, data integrity is not in question. Typical error correction schemes normally can handle single bit randomly distributed errors in readback. However, there are times when the density of errors is so great that data rereads would be required, which would degrade performance. In a worst case, heads or drives may have to be replaced in the field to attempt to eliminate the problem.
It can be seen then that there is a need for an improvement to prevent errors if and when the spikes occur.